Procedure for producing structures with globular primary crystals

ABSTRACT

Globular primary crystals are formed in an alloy by passing the alloy through a temperature zone between the liquidus and solidus temperatures at a velocity such that a temperature gradient G described by the relationship:

United States Patent Sahm Apr. 9, 1974 PROCEDURE FOR PRODUCING 3.542.541 11/1970 Lemkey 75/134 P STRUCTURES WITH GLOBULAR PRIMARY 3,552,953 l/l97l Lemkey et al. 75/l76 X 3,564,940 2/1971 7 Thompson et al. 75/171 [75] Inventor: Peter Sahm, Nussbaumen,

Switzerland [73] Assignee: BBC Brown, Boveri & Company,

Limited, Baden, Switzerland [22] Filed: Jan. 19, 1972 [21] Appl. No.: 219,098

[30] Foreign Application Priority Data Jan. 21, l97l Switzerland 970/71 [52] US. Cl. 75/65 ZM [51] Int. Cl C22b 9/02 [58] Field of Search. 75/65 ZM, 135, 134 R, I34 F, 75/ I76, 171

[56] References Cited UNITED STATES PATENTS 2,938,820 5/1960 Turner 148/134 Ar 6 l CRYSTALS Primary E.\'aminerL. Dewayne Rutledge Assistant E.\'aminerM. .I. Andrews Attorney, Agent, or Firm0blon, Fisher, Spivak, Mc- Clelland & Maier [57] ABSTRACT Globular primary crystals are formed in an alloy by passing the alloy through a temperature zone between the liquidus and solidus temperatures at a velocity such that a temperature gradient G described by the relationship:

G/V m/D (C C,,,) is established in the melt in front of the freezing surface, wherein G, V, m, D, C p and C are as defined herein below.

7 Claims, 5 Drawing Figures SHEET 1 [1F 3 PATENTEU APR 9 I974 SHEET 3 BF 3 BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing globular primary crystals in alloys of the type composed of at least two components, which are at most only mutually soluble in the solid state to a very limited degree, and in which one of the components ordinarily solidifies in faceted form, and wherein the concentrations of the alloy components are between the eutectic concentration and the concentrations corresponding to the maximum solubility.

2. Description of the Prior Art Globular solidification of primary crystals, i.e., approximately spherically shaped crystals which first separate out of the melt, have been attained with alloys of this type by melting techniques or by annealing below the eutectic or eutectoid temperature. In the latter method, one component is precipitated in lamellar form and is caused to coagulate. Neither of these prior art techniques however, have been completely satisfactory for all industrial applications.

When the primary crystals solidify from the alloy melt in faceted form, i.e., bevel-edged crystals, the working tool resulting therefrom will be considerably more brittle than if this component is solidified in a globular form. In fact, it is well known that spherical graphite pig iron has a considerably higher ductility than lamellar graphite pig iron.

New methods for preparing workpieces having globular primary crystals therefore have been sought.

SUMMARY OF THE INVENTION is established in the melt in front of the freezing surface.

In the above relationship:

G temperature gradient V velocity or speed of movement of the alloy work- 1 piece through the elevated temperature zone m rate of liquidus temperature rise C alloy composition D diffusion coefficient C. eutectic composition BRIEF DESCRIPTION OF THE DRAWINGS The invention will further be explained by reference to the drawings and photomicrographs, designated as FIGS. 1 through 3 in which:

FIG. I is a phase diagram of a binary or pseudobinary alloy consisting of components A and B;

FIG. 2 is a schematic diagram of a suitable annealing device, usable with this invention; and,

FIGS. 3, 4 and 5 are metallographic micrographs showing an example of a workpiece according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The phase diagram shown in FIG. I is of the type further described in W. Koster, et al., Archives of the Metallurgy oflron, (Archiv fur Eisenhuttenwesen) Vol. 26 (1955) pp. 555-559 and as described in F. D. Lemkey. Metallurgical Transactions 1 (1970) 2,799-2,806.

In the case of the eutectically solidifying alloy A-B in FIG. 1, component A in the solid state is soluble in component B up to a maximum concentration C while component B in the solid state is soluble in component A up to a maximum concentration C The eutectic concentration is denoted C in the diagram. The freezing temperature range is denoted AT in the hypoeutectic region and AT in the hypereutectic region. Components A and B must be selected as regards quality and quantity in such a waY that the concentration lies between the eutectic concentration C and the concentrations of maximum solubility in the solid state C A or C and such that the primary precipitating crystals solidify in facetted form. To change the structure at the selected part ofa workpiece consisting of an alloy A-B, the composition of which lies between the concentrations C C,. or C C n, the workpiece is heated to a temperature in the range of AT, or AT and then slowly cooled.'For this purpose, as shown in FIG. 2, the workpiece l is positioned in a double-walled quartz envelope and fixed at support-points 3 and 4 which are rigid with respect to the envelope. The wall of the envelope is water-cooled. The quartz envelope 2 is closed by a cover 5 having a connection 6 through which a protective gas, preferably argon, can be fed into the interior of the quartz envelope 2. Envelope 2 is also arranged to move along the longitudinal axis within an electrically heated resistance coil 7. The workpiece 1, together with quartz envelope 2 is drawn at velocity V through coil 7 and thus locally heated to a temperature within the range AT, or AT,,. By suitably selecting temperature Twithin the temperature range AT or AT or by varying the velocity V, it is possible to alter the geometric dimensions of the globular primary crystals formed. Velocity V must be so chosen that in the cooling phase a temperature gradient G according to the relationship becoming established. The symbols 6, V, m, D, C, and C, are as defined above.

Alloys which are treatable by the methods of this invention include the quasi-binary alloys Co, Cr, Cr Co C wherein the range of X 0.6 to 1.6 or of the type Me,,Cr- Cr Me C wherein Me represents the elements nickel, iron, cobalt or manganese, and particularly nickel or iron.

In one embodiment, the heating zone has a length of between 0.5 and 2.5 cm and the velocity is between 2 and 7 cm per hour.

Having generally described the invention a further understanding can be attained by reference to the following specific example which is presented for purposes of illustration only and is not intended to be limiting unless otherwise specified.

EXAMPLE To produce a rod-shaped workpiece of a quasi-binary alloy (Co Cr,, 48 percent vol. (Cr Co C 52 percent vol., 40.22 percent wt Co, 54.78 percent wt Cr and 5 percent wt C were melted in a A1 0 crucible and held for min at a temperature of 1,50() C. The melt was then drawn into a quartz tube. After cooling, the rod (workpiece), which had solidified in the quartz tube, was cleaned by etching for /2 hour in hydrofluoric acid to remove adhering Si 0 The workpiece was then brought to the annealing device described with reference to FIG. 2. The workpiece was drawn at a velocity of 7 cm/h through resistance coil 7 which has a heating zone 1.5 cm long. In a first experiment the temperature in the workpiece on passing through the heating zone was approximately 1,350 C. The temperature gradient was 50 C/cm or less. The irregular structure of the workpiece cooled from the molten state in the quartz tube can be seen in FIG. 5. FIG. 4 shows the fine globular structure obtained by annealing at at 1,350 C. FIG. 3 shows the coarse globular structure obtained by annealing at 1,400-C.

ln this example:

G 50 C/cm or less (S 30 Vol. percent C 52 Vol. percent A oJ 'iLa) B s.2 o.s a)

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention. Accordingly,

What is new and desired to be secured by letters patent is:

l. A process for producing globular primary crystals in an alloy containing at least two components, at least one of which solidifies in facetted form, and which in the solid state are mutually soluble to at most a limited degree, and wherein the concentration of the alloy components is between the eutectic concentration and the concentration corresponding to the solubility maxima, which comprises continuously moving a workpiece of an alloy through an elevated temperature zone wherein said workpiece is heated to a temperature between the solidus and liquidus temperatures of the alloy, and then continuously moving said workpiece out of said elevated temperature zone, and cooling said workpiece so that a freezing surface is established in front of the melt and wherein the velocity (V) of the movement of the workpiece through the elevated temperature zone, and the temperature gradient G established in themelt in front of the freezing suface are controlled by the relationship wherein m is the rate of rise in temperature of the liquid phase of the melt, D is the diffusion coefficient, C is the concentration of one constituent of the eutectic in the eutectic composition, and C is the concentration of the corresponding constituent in the alloy composition of the workpiece being treated, thereby producing globular primary crystals.

2. The process of claim 1 wherein the'alloy used is a hypereutectic alloy.

3. The process of claim 2 wherein said hypereutectic alloy is the quasi-binary alloy (Co Cr 48 percent vol. (Cr Co C 52 percent vol.

4. The process of claim 3 wherein the approximate length of the elevated temperature zone is 0.5 2.5 cm.

5. The process of claim 3 wherein the temperature is at least l,350 C.

6. The process of claim 5 wherein the temperature is 1',400 C.

7. The process of claim 1 wherein the velocity is between 2 and 7 cm per hour. 

2. The process of claim 1 wherein the alloy used is a hypereutectic alloy.
 3. The process of claim 2 wherein said hypereutectic alloy is the quasi-binary alloy (Co0.7Cr0.3) 48 percent vol. -(Cr6.2Co0.8C3) 52 percent vol.
 4. The process of claim 3 wherein the approximate length of the elevated temperature zone is 0.5 - 2.5 cm.
 5. The process of claim 3 wherein the temperature is at least 1, 350* C.
 6. The process of claim 5 wherein the temperature is 1,400* C.
 7. The process of claim 1 wherein the velocity is between 2 and 7 cm per hour. 